Many wireless communication systems now support multiple kinds of services, including for instance circuit switched voice services, packet data services, high data rate services, etc. These different services have very different characteristics. Moreover, different applications using the same general service may nonetheless impose different demands on that service. For instance, an internet browsing application may be supported by a packet data service that has a variable delay and throughput, while a multimedia streaming application may be supported by a packet data service having a relatively constant average throughput and low delay.
A wireless communication system supports these varying services through the use of radio bearers. A radio bearer supports the transfer of data, e.g., user data, over a radio connection between a wireless communication device and a base station with defined data transfer characteristics (e.g., with a defined quality of service, QoS). Different radio bearers are configured to provide different defined transfer characteristics.
Under some circumstances, though, the configuration or state of a given radio bearer may need to be changed, e.g., in order to optimize the radio bearer for the current requirements of the wireless communication device. A change in the configuration or state of a radio bearer involves, as non-limiting examples in a context where the system is a High Speed Packet Access (HSPA) system, adding or removing the radio bearer, moving the radio bearer between a dedicated physical channel (DPCH) and enhanced uplink (EUL)/high speed (HS), changing the spreading factor and/or bit rate, and/or adding or removing connection capabilities (e.g., EUL 2 ms/10 ms TTI, Dual Cell or multi-carrier, 64QAM, MIMO, CPC, DL enhanced L2, UL improved L2).
Consider the specific example of a radio bearer configuration change relating to a change in the transmission time interval (TTI) of a radio bearer. The TTI is a radio bearer parameter that defines the interval of time in which a transmission occurs over the air interface. In some systems, for instance, a set of one or more so-called transport blocks are fed from a medium access control (MAC) layer to the physical layer, and the TTI is the time it takes to transmit that set of one or more transport blocks over the air interface.
Regardless, a longer TTI (e.g., 10 ms or above) proves more robust in the face of poor channel conditions. On the other hand, a shorter TTI (e.g., 2 ms) reduces latency, which is required to provide good end-user experience when supporting mobile broadband services. Because of this, it is desirable to use a shorter TTI over as wide an area as possible. However, at least in current 3G networks, a substantial number of large macro cells still exist. With a macro cell being so large, it generally proves challenging for the cell to support a TTI as short as 2 ms over its entire coverage area. In such environments, it may be necessary to fall back to a longer TTI, e.g., 10 ms, when a wireless communication device approaches the cell boundary. This however requires that a radio bearer configuration change be triggered when the device approaches the cell boundary, and that the change be applied.
Regardless of the particular type of radio bearer configuration or state change, triggering and applying this change at an optimal time proves important for ensuring high system performance. In order to trigger and apply a radio bearer configuration change at the optimal time, the criteria used to trigger the change should be accurate and the procedure used to actually apply the change should be fast and robust.
With regard to the criteria used to trigger the change, at least some radio bearer configuration changes (like the TTI switch described above) are triggered depending on the uplink coverage of a wireless communication device. Known approaches measure this coverage as a function of how long the device operates at maximum output power. When the device operates at maximum output power for a certain amount of time (the time-to-trigger, TTT), an event (e.g., Event 6d in HSPA EUL) is triggered. This TTT is configured by a node in the network, e.g., a radio network controller (RNC). When the RNC receives this event from the device, it considers the device to be running out of coverage and triggers a radio bearer configuration change.
With regard to the procedure used to implement a radio bearer state or configuration change, different procedures can be used depending on whether the source and target configuration/state are compatible. If they are compatible, then both the device and base station may be able to apply the change at different times (i.e., non-synchronously) without the radio connection failing. On the other hand, if they are not compatible, then the device and base station should apply the change at the same time (i.e., synchronously) in order for the radio connection to survive.
In known approaches to synchronous application of a radio bearer state or configuration change, a higher-layer (e.g., a Radio Resource Control, RRC, layer or layer 3) centrally coordinates application of the change to occur synchronously at the wireless communication device and base station. A higher-layer message, for instance, is sent from a radio network controller (RNC) to both the device and base station ordering the change and specifying a future point in time (called “activation time”) at which the change is to be applied synchronously. This activation time is defined by a connection frame number (CFN). The CFN is a counter 0 . . . 255 (known by RNC, base station and device) which is stepped every radio frame (every 10 milliseconds) and thus has a wrap around every 2.56 seconds (256*10 ms). The RNC will decide on how far ahead the activation time shall be set based on the expected time to forward the change order message to the device and the base station. Typically the time to forward the order message via the air interface to the device is the limiting factor. Indeed, due to occasional loss of this message and its acknowledgement, the activation time must be set conservatively (i.e., longer) to allow for several retransmissions. That said, the range of the CFN dictates that the RNC cannot set the activation time to be more than 2.56 seconds (minus some margin) ahead. If this time is not enough to successfully forward the order to the device, the change will typically fail and the call is dropped.